Explain how reproductive isolation leads to speciation, and describe adaptive radiation and mass extinction as patterns in the history of life.
The biological species concept defines a species as a population whose members can interbreed and produce fertile offspring under natural conditions -- a genuinely useful working definition, though honestly limited, since it does not apply to asexually reproducing organisms and cannot be tested directly on extinct species known only from fossils, so biologists also rely on shared anatomical or genetic traits in those cases. Speciation, the formation of new species, requires reproductive isolation: populations must stop interbreeding for long enough to diverge genetically. Allopatric speciation, separation by a geographic barrier such as a canyon, a mountain range, or a new island population, is the most commonly documented pathway; sympatric speciation, divergence without geographic separation, is rarer, occurring for example through polyploidy, an error producing extra full sets of chromosomes, in some plant lineages.
At the deepest time scales, two large-scale patterns recur throughout the fossil record. Adaptive radiation is the rapid diversification of one ancestral species into many descendant species, each adapted to a different available niche -- Darwin's finches diversifying across the Galapagos Islands, or mammals diversifying rapidly after the extinction of the non-avian dinosaurs, are two well-documented examples. Mass extinction events, of which five major ones are identified in the fossil record (the end-Cretaceous event, 66 million years ago, which ended the age of non-avian dinosaurs, is the most well-known), sharply reduce diversity but also tend to open ecological opportunities that surviving lineages then radiate into, linking mass extinction and adaptive radiation as two sides of the same long-term pattern.
Mastery looks like: They can explain why reproductive isolation is required for speciation, distinguish allopatric from sympatric speciation, and describe the connection between mass extinction and adaptive radiation.
Common stumbles: Assuming speciation happens quickly or all at once rather than gradually, as populations slowly accumulate genetic differences; confusing adaptive radiation, diversification, with mass extinction, loss of diversity, as opposite ends of the same larger pattern.